Review the original item:
For two weeks, Todd refused to go to his accounting class because Professor
Gastineau would not allow students to wear hats,
under no circumstance would Todd allow anyone to see how badly the
barber had butchered his hair.
You wanted to fix it this way:
For two weeks, Todd refused to go to his accounting class because Professor
Gastineau would not allow students to wear hats,
therefore under no circumstance would Todd allow anyone to see how
badly the barber had butchered his hair.
Adding therefore will not fix the comma splice. Therefore is a conjunctive adverb and has no special power when you are joining two main clauses.
You might want to consult the rules for fixing comma splices and fused sentences.